Fix startup-css-check as-if-foss rules

.gitlab/ci/frontend.gitlab-ci.yml

.stylelintrc

app/assets/stylesheets/pages/login.scss

app/assets/stylesheets/pages/search.scss

app/assets/stylesheets/startup/startup-dark.scss

app/assets/stylesheets/startup/startup-general.scss

app/assets/stylesheets/startup/startup-signin.scss

app/views/layouts/_head.html.haml

app/views/layouts/_startup_css.haml

app/views/layouts/devise.html.haml

ee/app/assets/stylesheets/startup/startup-dark.scss

ee/app/assets/stylesheets/startup/startup-general.scss

ee/app/assets/stylesheets/startup/startup-signin.scss

package.json

scripts/frontend/startup_css/constants.js

scripts/frontend/startup_css/startup_css_changed.sh

scripts/frontend/startup_css/write_startup_scss.js

spec/frontend/fixtures/startup_css.rb